1. Field of the Invention
The present invention relates to a surface acoustic wave device used for telecommunication equipment.
2. Description of the Related Art
A surface acoustic wave device, used for conversion between an electric signal and a surface acoustic wave (SAW), comprises interdigital transducers (IDTs) formed of electrode fingers interdigitated on a piezoelectric substrate. Among various kinds of surface acoustic wave devices, a surface acoustic wave resonator (SAW resonator), in particular, has advantages such as compactness, light weight and no-adjustment requirement, and is in widespread use as a device for telecommunication equipment.
Referring to FIG. 1, there is shown a plan view of a basic structure of a conventional SAW resonator. On a piezoelectric substrate 101, an IDT is formed of a plurality of electrode fingers 102 arranged in an interdigital configuration, a pair of bus bars 104 which are opposed to each other and connected with the electrode fingers 102 in an alternate fashion, input/output ports 105 and 106, and a plurality of fingers 107 which are opposed to the electrode fingers 102 on the open node side thereof and connected with each of the opposing bus bars 104 to provide a reflector function. When a high-frequency electric signal is applied across the input/output ports 105 and 106, an electric field is produced between the electrode fingers 102 arranged in the interdigital configuration to excite surface acoustic waves on the surface of the piezoelectric substrate 101. In surface acoustic wave excitation, an excited surface acoustic wave having a wavelength identical to an interdigital pattern period P of the electrode fingers 102 and a surface acoustic wave having a wave number vector parallel to the direction of arrow 103 are excited most intensely since they are in phase on an electrode finger crossover area. In the SAW resonator shown in FIG. 1, a surface acoustic wave leaks out of the IDT through both sides thereof to cause a large energy loss, resulting in a low Q value in resonance.
Referring to FIG. 2, there is shown an exemplary electrode configuration of a conventional SAW resonator designed for Q-factor improvement over the conventional SAW resonator in FIG. 1 (proposed in Japanese Unexamined Patent Publication No. H6 (1994)-85602 and xe2x80x9cSmall-Size Love-Type SAW Resonators with Very Low Capacitance Ratioxe2x80x9d by Hiroshi Shimizu and Yuji Suzuki xe2x80x94The Transactions of the Institute of Electronics, Information and Communication Engineers, A Vol. J. 75-A NO. 3 pp. 458-466, March 1992). In this exemplary configuration in which a surface acoustic wave crossing over the electrode fingers 102 is excited on a rhombic area 108 (excitation area) enclosed by the broken line, apodization is made in a fashion that the cross lengths W of the electrode fingers 102 are maximum at the center of the IDT and zero at both ends thereof, thereby reducing a degree of spurious response. Further, leakage of a surface acoustic wave out of the IDT through both sides thereof is reduced since the excitation area 108 is narrowed on both sides of the IDT and surface acoustic wave reflection is made by a reflector 109 comprising the electrode fingers 107 which are so arranged on the periphery of the excitation area 108 as to oppose the electrode fingers 102 in a grating form. Thus, an energy loss can be decreased to improve the Q factor. Note that in addition to the electrode fingers 107 functioning as elements of the reflector 109, parts of the electrode fingers 102 disposed on the periphery of the excitation area 108 also serve as reflector elements. That is to say, some parts of the electrode fingers 102 are used for excitation and the other parts of the electrode fingers 102 are used for reflection, depending on the locations thereof.
In the SAW resonator having the electrode configuration shown in FIG. 2, the electrode fingers on both sides of the IDT are opposed mutually in parallel. Therefore, the electrode fingers on both sides of the IDT reflect a surface acoustic wave component having a wave number vector parallel to the direction of the arrow 103 (inharmonic higher-order longitudinal mode component), causing a standing wave having a waveform such as 201. Furthermore, in the SAW resonator having the electrode configuration shown in FIG. 2, the boundaries between a region of the reflector 109 and the bus bars 104 are opposed mutually in parallel. Therefore, a surface acoustic wave component having a wave number vector perpendicular to the direction of the arrow 103 (inharmonic higher-order transverse mode component) is reflected on the boundaries between the reflector 109 and the bus bars 104, causing a standing wave having a waveform such as 202. These standing waves produces spurious response in an impedance characteristic of the SAW resonator.
FIG. 3 shows an example of an impedance characteristic of the conventional SAW resonator shown in FIG. 2. The conventional SAW resonator in FIG. 2 is fabricated in the following manner: On a piezoelectric substrate made of 15xc2x0-rotated Y-cut X-propagation lithium niobate (hereinafter referred to simply as 15xc2x0 YX-LN), aluminum is deposited by evaporation method, and an IDT electrode pattern is formed by photolithography and dry etching method. In FIG. 3, reference numeral 112 indicates a peak corresponding to a series resonance frequency of the SAW resonator. As shown in this figure, a multiplicity of spurious response peaks occur in a lower-frequency region 113 with respect to the series response frequency. These multiple spurious response peaks give rise to considerable problems, particularly in a case where the SAW resonator is employed as an oscillation element in a voltage controlled oscillator (VCO). Where the SAW resonator is employed as a VCO oscillation element, an expansion coil is connected to the SAW resonator and the lower-frequency region 113 with respect to the series resonance frequency thereof is used for resonant oscillation. Since spurious response in a VOC oscillation frequency region incurs a frequency discontinuity state, the spurious response peaks in the lower-frequency region 113 are critically problematic in operation.
Referring to FIG. 4, there is shown an exemplary electrode configuration of a conventional SAW resonator designed for improvement over the conventional SAW resonator in FIG. 2 (proposed in xe2x80x9cHigh-Q Wide-band SAW Resonators for VCOxe2x80x9d by Atsushi Isobe et al.xe2x80x94Proceedings of the 20th Symposium on Ultrasonic Electronics, pp. 63, November 1999). In this example, the inside face of each of the bus bars 104 is formed in parallel with the periphery of the excitation area 108 so that a phase of a standing wave and a frequency incurring a standing wave are unrelated to the propagation direction of a surface acoustic wave for suppression of spurious response.
Although the conventional SAW resonator shown in FIG. 4 is successful as far as the suppression of spurious response is concerned, it is unsatisfactory for use as a VCO oscillation element. FIG. 5 is a graph indicating an exemplary impedance characteristic of the conventional SAW resonator shown in FIG. 4. In fabrication of the conventional SAW resonator in FIG. 4, aluminum is deposited on a piezoelectric substrate made of 15xc2x0 YX-LN by evaporation method, and an IDT electrode pattern is formed by photolithography and dry etching method.
In the conventional SAW resonator in FIG. 4, although a flat impedance characteristic having virtually no spurious response is attained in a lower-frequency region with respect to a peak 112 corresponding to a series resonance frequency thereof, a ripple 111 exists around a frequency of 207 MHz. Where the conventional SAW resonator in FIG. 4 is used as a VCO oscillation element, an oscillation frequency discontinuity occurs in the vicinity of the frequency corresponding to the ripple 111. For this reason, the conventional SAW resonator in FIG. 4 is not applicable as a VCO oscillation element in a frequency band including the frequency corresponding to the ripple 111.
It is therefore an object of the present invention to provide an SAW resonator which is capable of suppressing occurrence of a ripple in an impedance characteristic thereof for application to wideband VCO operation.
Recognizing that the ripple 111 in the impedance characteristic shown in FIG. 5 is caused by the scattering of a surface acoustic wave on the reflector 109 of the SAW resonator in FIG. 4, the inventors herein propose a technique for suppressing occurrence of the ripple by disposing the electrode fingers 107 constituting the reflector 109 in an arrangement relatively shifted with respect to the electrode fingers 102 on the excitation area 108 or by optimizing the lengths of the electrode fingers 107 constituting the reflector 109.
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.